Apparatus for measuring characteristics of materials through the application of pulses of successively increasing amplitude



Jan. 3, 1967 As 3,296,523

APPARATUS FOR MEASURING CHARACTERISTICS OF MATERIALS I THROUGH THEAPPLICATION OF PULSES 0F SUCCESSIVELY INCREASING AMPLITUDE I OriginalFiled Aug. 29. 1960 PORTON UP TO HORIZONTAL ll IE T PULSE SWEEP SYSTEM Y-OSCILLOSCO' PU S R AM LITu PE E CIONTIRI L NORMAU'Y" INIZUT v ,I3

SAMPLE CLIPPER I4 CURRENT MEASURING RESISTOR 2O FROM AMPLITUDE CONTROLI5OK 2Q DELAY 22 0.25 F.D.C. BLOCKING "T- OAPAOITATOR v Q swEEP 21 swEEPi GENERATOR 8 AMPLIFIER To 25 2 OEFLEcRg HORIZONTAL C'RCU POSITIONING ATIT-i=3 I l I l Z O x=O x=o X=O x=o x=O ((1) (b) (c) (d) (e) CLIPPERCLIPPER swEEP HORIZONTAL VOLTAGE INPuT OUTPUT DELAYED PuLsE BEINGINsERTEO CHANGED INVENTOR I x A. A 'OSCILLOSCOPE TRAOE GEORGE HAATTORNEY United States Patent O 3,296,523 APiARATUS FOR MEASURHNGCHARACTER- ISTlCS OF MATERIALS THRGUGH THE APPLI- CATION OF PULSES FSUCCESSIVELY IN- QREASING AMPLITUDE George A. Haas, 5 Fort Hill Drive,Alexandria, Va. 22310 Continuation of application Ser. No. 52,745, Aug.29, 1960. This application July 17, 1963, Ser. No. 296,422 2 Claims.(Cl. 324-57) The invention described herein may be manufactured and usedby or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

This application is a continuation of application Serial No. 52,745, nowabandoned, filed August 29, 1960.

This invention relates to apparatus for measuring electricalcharacteristics of materials to study short term equilibrium conditionsand yet avoid long term changes in the materials.

In the measurement of characteristics of materials it is customary todetermine the behavior of such a material when forces of differentamplitudes are applied. Although such measurements are in no way solimited, at typical application of such measurement is in thedetermination of the resistance characteristics of an electricalcomponent such as an electron tube, a transistor, a conductive solution,or many other forms. In such devices it is desired to obtain thecharacteristics over the range of concern in as short a time as possibleto minimize the effect of long term variables, however it is essentialthat the measurements 'be taken at equilibrium conditions even thoughthey may persist for a short time. When measurements are taken in otherways than the combination of short term equilibrium conditions withavoidance of long term changes, various undesirable effects are notedsuch as overheating of the sample, donor migration, polarizationeffects, electrolysis effects and various other thermal, electrical,mechanical and chemical changes in the sample.

In the general, the prior art contains two methods of makingmeasurements such as the foregoing, however these do not possess thedesired characterization of short term equilibrium and avoidance of longterm changes. A first method employed is one in which a voltage pulse isapplied to a sample (semiconductor, thermionic diode, electrolyticsolution, etc.) and then the pulse voltage and pulse current arecarefully measured on separate Oscilloscopes. In this method the majordifficulty is the lengthy time necessary to measure the sample asdetermined by the time required to obtain accurate current and voltagereadings on separate Oscilloscopes multiplied by the number of suchreadings desired. This often is long enough to allow changes to occur inthe sample such as drift in temperature, activity, and so forth. Anotherobjection is that if the pulse is not perfectly flat, an error mightarise from reading the voltage and current at slightly different pointsalong the pulse.

A second method of measuring such voltage-current characteristics is toapply a sawtooth voltage to the sample and present a plot of current asa function of voltage on one oscilloscope. This method overcomes some ofthe foregoing difliculties stated in connection with the first prior artmethod of excessive measuring time since it plots current as a functionof voltage directly which can then be photographed. However, inherent inthis technique is the disadvantage that the voltage across the sample isconstantly changing. Therefore, the accuracy is limited by thesuperposition of a displacement current on the regular current to bemeasured. This displacement current is given by do (a v) i where C isthe capacity of the sample in farads and dv/dt is the slope of theapplied sawtooth in volts per second. It can be seen that this problembecomes more pronounced at higher values of C and for shorter durationof the sawtooth wave form (higher dv/dt). Another difficulty broughtabout by this method is that the system under measurement is not inequilibrium and therefore problems requiring short term equilibriumconditions (e.g., effect of carrier diffusion or recombination, emissiondecay, etc.) cannot be easily undertaken. The technique to be describedeliminates these difiiculties and is capable of giving pulsedvoltage-current characteristics in a matter of a few seconds that can bemeasured with an accuracy of the order of tenths of one percent.

It is accordingly an object of the present invention to provideapparatus for measuring characteristics of materials wtih a high degreeof accuracy.

Other and further objects and many of the attendent advantages of thisinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein:

FIGURE 1 shows a typical combination of apparatus in accordance with theteachings of the present invention.

FIGURE 2 shows in greater detail a typical signal input circuit for theapparatus of FIGURE 1.

FIGURE 3 shows various wave forms present in the apparatus of FIGURE 1under various conditions.

In accordance with the basic teachings of the present invention, amethod of measuring characteristics of materials is provided whereinrectangular pulses of energy are employed and applied across a sampleunder test, measurement being made of the energy flow through the sampleduring the pulse and by means of a suitable indicator such as anoscilloscope determining the relationship of the applied pulse force andthe pulse energy flow through the sample. This is accomplished byapplying to the horizontal sweep system (superimposed on the regularhorizontal time base) a portion of the energy pulse which appears acrossthe sample, the horizontal sweep being synchronized to the pulsefrequency. The pulse energy flow through the sample is obtained in theelectrical case from the voltage developed across a measuring resistor.This voltage is then clipped so that the high peaks from thedisplacement current at the leading and trailing edges of the pulse willnot not saturate the scope amplifier. The output of the clipper is thenapplied to the vertical deflection system of the oscilloscope in theusual manner for display of input signals. With the pulse voltage of theelectrical case applied to the horizontal sweep system of theoscilloscope as well as to the sample, a peculiarly displaced form ofindividual presentation is obtained; however, since the horizontaldeflection of the start of the sweep is proportional to the magnitude ofthe pulse voltage also at a selected time after the start of the pulse,it corresponds to the identical point on the pulse that causes thedeflection in the current reading. This automatically eliminates thedanger of error arising from not reading the voltage that directlycorresponds to the current produced thereby. With the apparatus andtechnique established as in the foregoing, the pulse voltage is variedwith a series of curves being made which can be recordedphotographically in which a line will be traced out corresponding to thepulsed voltage-current characteristics of the sample at a selected timeafter the start of each pulse. The number of points obtainable in theline traced is equal to the repetition rate times the time employed orrequired to raise the pulse voltage, therefore enough points aregenerally obtained in a few seconds so that the locus of the trace formsa continuous line. This time is short enough to eliminate most of theerrors arising from the longer term drifts in temperature, activity, andso forth previously mentioned. By simply changing the delay time, it ispossible to select different points along the pulse for which thecurrent-voltage characteristics might be desired.

With reference now to FIG. 1 of the drawing the apparatus shown thereincontains a pulse generator 10, an amplitude control device 11 nec d th rto by means of which selected amplitudes of the pulse output signal maybe applied to two output lines, an oscilloscope 12 connected to oneoutput of the amplitude control device 11, a sample 13 connected to oneoutput of the amplitude control device 11, said sample being connectedin series wtih a resistor 14 by means of which a voltage is producedproportional to the current passing through the sample, a clipper 16connected across the current measuring resistor 14 and to theoscilloscope 12.

The connection of the amplitude control device 11 to the oscilloscope 12is to the horizontal input of the scope where it is applied and mixedwith the normal horizontal sweep signal forming the x time base thereof.The clipper 16 is connected to the normal Y or vertical input of the voscilloscope 12.

The duration of the pulse from pulser is selected of any convenientduration; typically, however, it is one hundred microseconds, it beingdesired that the pulse be of a flat top variety so that there is nosubstantial voltage variation during the pulse itself. The wave form ofFIG- URE 3a shows in general the character of the current flow throughthe sample indicating, as a result of the voltage applied thereto, aninitial spike portion corresponding to the displacement current. Theamplitude control device 11' is of any suitable conventional nature; itmay be simply a manually controlled potentiometer arrangement andincluding auxiliary amplifiers by means of which the amplitude of theoutput signals applied to the oscilloscope and to the sample may bevaried as desired. The amplitude of the signal applied to theoscilloscope is selected to provide a convenient deflection. Typicallythe maximum horizontal deflection for a five inch scope would be in theorder of 2 and /2 to 3 inches, while a vertical deflection of the orderof l and /2 to 2 inches would be appropriate.

To eliminate undesired effects of the displacement current it ispreferred that the oscilloscope 12 contain either within it or in theform of separate components, a delay device (described more fully inrelation to FIG. 2) which delays the start of the main sweep of theoscilloscope 12 following the occurrence of the pulse delivered to theoscilloscope 12 from amplitude control 11. Thus the horizontal sweep ofthe oscilloscope 12 will not start until the passage of a period of timefollowing the pulse so that the initial portion of the pulse is notpresent in the clipper output, being modified in such a way that thepresentation of the Y signal on the oscilloscope is a short durationpulse of substantially rectangular characteristics shown by wave form30. The initial portion of the pulse thus occurs during flyback of thescope or otherwise in conventional manner occurring duringnon-intensified portions of the cathode ray tube presentation. Thefurther application of the pulse from the amplitude control device 11 tothe horizontal sweep system of the oscilloscope 12 results in adistortion of the pulse appearing on the scope 12 to where therectangular pulse of FIGURE 30 is not actually viewed but rather a pulsedisplaced in portions thereof as shown by FIGURE 311 is obtained. Theinitial high amplitude portion of the pulse of the FIG. 30 is displacedto the right by the application of the pulse signal to the horizontalsweep system to where it appears to be displaced in terms of a quantityL which is proportional to the amplitude of the pulse applied to sample13. Thus the indication of the oscilloscope 12 contains a verticaldisplacement proportional to the amount of current flowing through thesample, the voltage applied to the sample being indicated as a magnitudeproportional to the magnitude of the quantity L of FIGURE 3d.

The result of this operation is that when the magnitude of the pulseoutput of'the amplitude control device 11 is varied a series ofdistorted displaced pulses is obtained as shown in FIGURE 3c in whichthe leading edge of the pulse will appear along the locus of the linea-b which is exactly indicative of the voltage-current characteristicsof the sample under test at the selected instant in time following theapplication of the pulse voltage. This device is capable of analyzingvery short term equilibrium conditions in that by merely selecting themagnitude of T occasioned by the delay in the start of the horizontalsweep of the oscilloscope 12 it is possible to select the instant intime for each pulse following which the measurement is taken. It is thuspossible to obtain for all pulses an indication of the current flowingthrough the circuit after the initial displacement current has subsidedor even during it, if actually desired so that complete uniformity ofoperation is obtained.

With the pulser 10 operative at a fairly low rate, typically 60 cyclesper second, it is possible to obtain a complete sweep of the line a-b ofFIGURE 3b in a matter of a few seconds by varying the amplitude controldevice 11. This variation can be by hand manipulation of a potentiometeror by some mechanical or electrical device. With camera 17 then set upso as to photograph the presentation of the oscilloscope face, a veryrapid and permanent record of the operation is obtained from which thecharacteristics of the sample may be determined.

FIGURE 2 shows the manner in which the variable amplitude output ofamplitude control device 11 may be applied to the normal deflectioncircuit of a typical oscilloscope. In FIGURE 2, the output of theamplitude control 11 is connected to terminal 20 which, in turn, isconnected through a typical K resistor 21 and a DC. blocking capacitor22 to the normal connection between the sweep generator 23 of theoscilloscope and the sweep amplifier 24 of the oscilloscope. Between thecondensor 22 and the sweep generator 23 an isolating and voltagedividing resistance 25 is inserted to provide not only isolation butalso to provide voltage divider action with resistance 21 whereby aselected por- -tion of the output from amplitude control device 11 iseffectively applied to sweep amplifier 2 4. Sweep amplifier 24 output isapplied as normal via line 26 to the conventional deflection circuitryof the oscilloscope 12. In addition, a conventional horizontalpositioning apparatus for the oscilloscope 12 is indicated by block 27.Synchronism of the sweep generator 23 to the frequency of pulser 10 isprovided by connection of the input signal through delay device 28 tothe synchronization or trigger portion of the sweep generator 23. Delaydevice 28 is made adjustable, preferably, to permit a control over theduration of the time delay of FIGURE 3b.

A large number of measurements made of the voltage currentcharacteristics of ohmic samples have been made with a high degree ofaccuracy employing the apparatus described in the foregoing. It isevident, however, that the pulse measuring system is not restrictedsolely to applications involving conductivity type measurements, butrather can be used for any two interdependent pulsed functions or eventwo interdependent functions where only one is pulsed. An example ofthis latter case is a plot of pulsed current as a function oftemperature for a given pulsed voltage. Here the DC. output of athermocouple can be amplified and inserted into the horizontaldeflection system as a DC voltage.

The specific application for which this pulse measuring system wasevolved was to obtain very accurate thermionic emission characteristicsof oxide cathodes in strong field regions. The normal D.C. methods foraccurate measurements are not adequate for this case, since a continuousfield acting on this semiconductor surface causes such unwanted effectsas poisoning and donor migrations to take place. Furthermore, because ofthe introduction of errors outlined in the preceding discussion, thestandard pulse measuring techniques of the prior art were incapable ofproviding the accuracy desired.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. Apparatus for measuring the characteristics of a sample of materialcomprising:

means for applying a series of rectangular single polarity pulses ofsuccessively increasing amplitude to said sample of material forproducing a current which passes through said sample,

means for receiving said current after passing through said sample andfor deriving an output proportional to the current flow through saidsample which is responsive to the successively increasing pulses.

display means having horizontal and vertical deflection means for signalpresentation in two coordinates, means in said display means forproducing a time base sweep signal synchronized with the successivelyincreasing single polarity pulses, means for applying the output signalproportional to the current flow through said sample to said verticaldeflection means of said display means, and means for applying theseries of successively increasing rectangular single polarity pulses andthe time base sweep signal to the horizontal deflection means of saiddisplay means.

2. Apparatus for measuring the characteristics of a sample of materialas set forth in claim 1 wherein the means for producing a time basesweep signal and the means for applying the continuously increasingrectangular single polarity pulses and the time base sweep signal to thehorizontal deflection means of said display means comprises:

a sweep amplifier coupled to said horizontal deflection means,

a sweep generator having a trigger input and an output,

said output coupled to said sweep amplifier,

delay means coupled between said sweep generator trigger input and saidmeans for applying said single polarity pulses, and

means for coupling said sweep generator output to said means forapplying single polarity pulses to said sample.

References Cited by the Examiner UNITED STATES PATENTS 2,371,636 3/1945McConnell 324 X 2,612,626 9/1952 Miles 324-26 2,616,058 10/1952 Wagner324--26 X 2,793,343 5/1957 Wagner 324-57 X 2,882,486 4/1959 Eberhardt324-26 2,900,582 8/1959 Moll 324-158 3,028,578 4/1962 Stanton 32468 XOTHER REFERENCES Spitzer et al., Measurement of the Lifetime of MinorityCarriers in Germanium," article in Journal of Applied Physics, vol. 26,No. 4, April 1955, pp. 414-417.

WALTER L. CARLSON, Primary Examiner.

FREDERICK M. STRADER, Examiner.

A. E. RICHMOND, Assistant Examiner.

1. APPARATUS FOR MEASURING THE CHARACTERISTICS OF A SAMPLE OF MATERIALCOMPRISING: MEANS FOR APPLYING A SERIES OF RECTANGULAR SINGLE POLARITYPULSES OF SUCCESSIVELY INCREASING AMPLITUDE TO SAID SAMPLE OF MATERIALFOR PRODUCING A CURRENT WHICH PASSES THROUGH SAID SAMPLE, MEANS FORRECEIVING SAID CURRENT AFTER PASSING THROUGH SAID SAMPLE AND FORDERIVING AN OUTPUT PROPORTIONAL TO THE CURRENT FLOW THROUGH SAID SAMPLEWHICH IS RESPONSIVE TO THE SUCCESSIVELY INCREASING PULSES. DISPLAY MEANSHAVING HORIZONTAL AND VERTICAL DEFLECTION MEANS FOR SIGNAL PRESENTATIONIN TWO COORDINATES, MEANS IN SAID DISPLAY MEANS FOR PRODUCING A TIMEBASE SWEEP SIGNAL SYNCHRONIZED WITH THE SUCCESSIVELY INCREASING SINGLEPOLARITY PULSES, MEANS FOR APPLYING THE OUTPUT SIGNAL PROPORTIONAL TOTHE CURRENT FLOW THROUGH SAID SAMPLE TO SAID VERTICAL DEFLECTION MEANSOF SAID DISPLAY MEANS, AND MEANS FOR APPLYING THE SERIES OF SUCCESSIVELYINCREASING RECTANGULAR SINGLE POLARITY PULSES AND THE TIME BASE SWEEPSIGNAL TO THE HORIZONTAL DEFLECTION MEANS OF SAID DISPLAY MEANS.